Controlled release of materials through membranes has been studied extensively. U.S. Pat. No. 2,572,669 and U.S. Pat. No. 2,661,991 disclose the release of a volatile solid, p-dichlorobenzene, through polyethylene polymer films of various thicknesses. U.S. Pat. No. 3,578,545 discloses release of odors from an absorbing material laminated to a porous, flexible plastic film; German patent publication 2,740,497, published Mar. 16, 1978, discloses the use of laminates to release pheromones. U.S. Pat. No. 4,017,030 discloses release of substances, including pheromones, using plastic capillary hollow fiber tubes, employing a stagnant gas layer at an open end as the "membrane". A similar approach is disclosed in EPO Publication 9,374, published April 2, 1980. U.S. Pat. No. 4,145,001 discloses package form release dispensers for deodorizers. Japanese Kokai JP 82/72904, published May 7, 1982, discloses release of "neat" pheromones containing small amounts of marker material to facilitate detection, from polyethylene capillaries. Other related disclosures include EPO Publication 31,719, published July 8, 1981, and EPO Application Publication No. 103,953, published Mar. 28, 1984. Additional release systems using cartridges inside permeable or impermeable containers, or, for concentrated liquids, envelopes of two superposed plastic layers, are disclosed in U.S. Pat. No. 3,770,199 and U.S. Pat. No. 3,785,556, respectively.
U.S. Pat. No. 3,951,622 to Wilk discloses a system for controlled release of the unchanged odor of a fragrance through polyethylene or polypropylene membranes. Frangrance is broadly defined, but must be a mixture of mutually soluble components, and the components must include an alcohol and an essential oil. The essential oil is a plant-derived volatile material. In this application, both the essential oil and alcohol constitute major ingredients and both are stated to be necessary in order to achieve putative synchronous release of all of the ingredients. The system is not reported to achieve control over the rate of release of any individual component, nor is it reported to offer uniform release over time. The release systems are not designed to maintain constant release over variations in temperature.
Control with respect to temperature and with respect to maintaining a desired constant rate of release over long time periods is significant in the design and success of releasing systems for certain applications, such as the release of substances in connection with insect control. In addition, release systems designed for commercial applications which require large numbers of devices must be economically constructed. The release must be persistent and consistent over extended times. It would also be desirable to obtain release of the desired material without the presence of an extraneous costly additive such as an essential oil. None of the methods or devices of the existent art are satisfactory for the purpose of low cost, high volume, pest control systems.